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Influence of the substrate on the electrical properties of thick-film resistors
The role of hybrids in LSI systems C. T. G O D D A R D
T. ISOGAL, I. T S U B O K A W A and H. OHTSU
IEEE Trans. Components, Hybrids Mfg Technology. Chmt-2(4), 367
(December 1979). New dielectric pastes with high and low dielectric constants and an advanced passivation system have been developed for highly reliable capacitors and crossovers in thick-film circuit modules. Colloidal magnetite has been found to be a good sintering agent in BaTiO3based high-K dielectric paste, and a crystallised low-K glass paste has been found to be suitable for small capacitors and crossovers. Alloyed Ag/Pd and Ag/PdO have been developed for conducting electrodes, which are compatible with the dielectric materials and the firing process. Physical defects are the critical factors in the glass passivation systems for the reliability because of possible moisture diffusion into the dielectrics. A new two-layered passivation system has been adopted. The capacitor system has been applied to an RF/I F thick-film hybrid module for radio-cassette recorders.
(December 1979). The cost of one square inch of ceramic circuit is about the same as one square inch of multilayer printed wiring board containing two signal layers with buried power and ground. But the interconneetion density attainable in hybrid is approximately ten times as high. By transferring interconnections from multilayer board to hybrids, the need for excessive layers in MLB is avoided or the total attainable gate count is increased. In comparison with single chip packages, the use of multichip hybrids provides a continuing competitive edge as the scale of integration in silicon is increased year by year. Large scale digital hybrid microcircuits for new military systems D. R. KLING and R. W. ILGENFRITZ IEEE Trans. Components, Hybrids Mfg Technology. Chmt-2(4), 372 (December 1979). The packaging density of digital circuits can be significantly increased by assembling multiple MSI, LSI and VLSI semiconductor chips in large hybrid microcircuits. The design, manufacturing and producibility factors for these large multilayered thick film hybrid microcircuits are described. These factors include: manual and computer aided design, materials, substrate processing, assembly, testing, and environmental screening. Recent production experiences in manufacturing three unique microcircuits containing T r L , C-MOS and ECL devices are discussed. Some problems and possible solutions for hybrid microcircuit reliability F. N. SINNADURAI, K. J. WILSON and D. W. J. B R A C E Microelectronics Journal 11 (1), (1980). Thick film hybrid microcircuits developed for various telecommunications applications and made by various technologies have been examined by overstress testing and by failure analyses, which have yielded a number of promising results and revealed a number of deficiencies. Examples are presented of the poor reliability obtainable not only from low-cost plastic-coated hybrids but also from supposedly superior hermeticallysealed hybrids - degradation occurring because of moisture permeation or desorption, the evolution of deleterious vapours, bond failure due to inter-metallic reactions, and so on. Reliability problems of this sort are often encountered when the technologies or manufacturing standards are not directed towards high reliability applications, despite conformity to accepted quality standards, so demonstrating that approval of manufacturing capability does not necessarily provide realiability assurance. Nevertheless, the reliability prospects for hybrids are quite bright, because problems can and have been overcome - for example, that some circuits even in low-cost encapsulations can approach the highest reliability standards, that the long term stability of thick-film resistors can be better than 1%, that bonding problems can be obviated by using the right interconnection materials and that less hazardous adhesives are avilable for use in hermetic encapsulations. The observations show that while it cannot be claimed that hybrid technologies are inherently reliable, neither does it appear that the opposite is t r u e - potentially high reliabilities beingachievable by exercising intelligent choice.
4.
Discrete Devices
Reversible breakdown voltage collapse in silicon gate-controlled diodes A. RUSU, O. P I E T R A R E A N U and C. BULUCEA Solid-St. Electron. 23, 473 (1980). A reversible breakdown voltage collapse is recorded in the high voltage range of the junction breakdown voltage vs gate voltage characteristic of silicon gate-controlled diodes, which is explained in terms of a spatial switching of the avalanche breakdown within the device structure. The collapse gate voltage is oxide-thickness dependent and is accurately predictable as the avalanche breakdown voltage of the deeply depleted MOS capacitor within the gatecontrolled diode structure. The minimum oxide thickness required for approaching the bulkdetermined breakdown voltage in field-plated planar diodes and transistors is found to range from 0.01 to 5.00 p.m for substrate impurity concentration from 1017to 5 x 101~cm-3, according to a design plot provided in the paper. Development of the thick-film capacitor and its application for hybrid circuit modules K. A B E , A. IKEGAMI, N. SUGISHITA. N. TAGUCHI,
IEEE Trans. Components, Hybrids Mfg Technology. Chmt-2(4), 434
TCR control of NI/Cr resistors R. A. THIEL and E. H. M A U R E R
IEEE Trans. Components, Hybrids Mfg Technology. Chmt-2(4), 467 (December 1979). A detailed study was made to identify the important deposition parameters and to determine how they interact to affect the TCR and sheet resistance of Ni/Cr films deposited on alumina. Wire-fed flash evaporation is described as a simple method fur the control of composition, deposition rate, and thickness. The Ni/Cr ratio, substrate temperature, amount of Au in the Ni/Cr, and the film thickness have major effects on TCR. Deposition rate and pressure and the effect of a Ni barrier layer between the Ni/Cr and the Au conductor were also investigated. The use of a Ni barrier layer degrades the resistor stability somewhat. Approximately 50% Cr is necessary to get the TCR on alumina below about + 100ppmpC, but variation from 50 to 70% has little further effect, while Au in the Ni/Cr depresses the TCR, and can make it negative. Reduced substrate temperature during film condensation and increased film thickness also reduce TCR. Sheet resistance is affected by thickness, Au content and substrate temperature. A particular value for each of the above parameters was found that when used together produced 'zero' TCR and 180f~/square sheet resistance. Influence of the substrate on the electrical properties of thick-film resistors A. CATI'ANEO, L. PIROZZI, B. MORTEN and M. PRUDENZIATI IEEE Trans. Components, Hybrids Mfg Technology. Chmt-3(l), 181 (March 1980). The thick-film technology is widely used for hybrid circuit manufacturing and there is a definite trend to use it in other applications. However, the understanding of the electrical, mechanical, and chemical characteristics of thick-film materials has been approached essentially on an empirical basis, even if many hypotheses have been formulated on their conduction mechanisms. In fact, thick-film resistors present a temperature coefficient of resistance that is negative at lower temperatures and positive at higher temperatures with a TCR vanishing in the range of the room temperature. Recent papers attribute the conduction mechanism to percolation tunneling of electrons through conductive grains embedded in the glassy matrix of the resistor layer. The model assumes that the resistance of the percolation paths dominates the resistance of a network electrically equivalent to the thick-film resistor. With this model a good fitting of the experimental data is obtained. However only data concerning Ru-based resistors screened and fired on 96% alumina substrates were considered. In order to better understand the influence of the ceramic substrates on the electrical and thermal characteristics of thick-film resistors, zirconia, beryllia, and alumina (with different purity) were employed in the present study. The results indicate that the 'substrate effect' plays an important role in ruthenium-based resistors, so that, in order to understand the thick-film conduction mechanisms, it is necessary to take into consideration the 'substrate resistor system' and not to limit the analysis to the film in itself. Of particular interest is the fact that the minimum of the resistance-versus-temperature CUlWevaries for different substrate materials, even if the resistors under test are made with the same resistor series and are characterised by the same sheet resistivity. An equation is proposed that correlates the resistor gauge factor to thermal expansion coefficient of the ceramic substrate. By assuming the validity of a recently proposed model of conduction mechanism in thick film, a new set of equations is proposed that fits the experimental results obtained on resistors screened and fired on substrates of different compositions or with a different content of impurities. 41
T. ISOGAL, I. T S U B O K A W A and H. OHTSU
IEEE Trans. Components, Hybrids Mfg Technology. Chmt-2(4), 367
(December 1979). New dielectric pastes with high and low dielectric constants and an advanced passivation system have been developed for highly reliable capacitors and crossovers in thick-film circuit modules. Colloidal magnetite has been found to be a good sintering agent in BaTiO3based high-K dielectric paste, and a crystallised low-K glass paste has been found to be suitable for small capacitors and crossovers. Alloyed Ag/Pd and Ag/PdO have been developed for conducting electrodes, which are compatible with the dielectric materials and the firing process. Physical defects are the critical factors in the glass passivation systems for the reliability because of possible moisture diffusion into the dielectrics. A new two-layered passivation system has been adopted. The capacitor system has been applied to an RF/I F thick-film hybrid module for radio-cassette recorders.
(December 1979). The cost of one square inch of ceramic circuit is about the same as one square inch of multilayer printed wiring board containing two signal layers with buried power and ground. But the interconneetion density attainable in hybrid is approximately ten times as high. By transferring interconnections from multilayer board to hybrids, the need for excessive layers in MLB is avoided or the total attainable gate count is increased. In comparison with single chip packages, the use of multichip hybrids provides a continuing competitive edge as the scale of integration in silicon is increased year by year. Large scale digital hybrid microcircuits for new military systems D. R. KLING and R. W. ILGENFRITZ IEEE Trans. Components, Hybrids Mfg Technology. Chmt-2(4), 372 (December 1979). The packaging density of digital circuits can be significantly increased by assembling multiple MSI, LSI and VLSI semiconductor chips in large hybrid microcircuits. The design, manufacturing and producibility factors for these large multilayered thick film hybrid microcircuits are described. These factors include: manual and computer aided design, materials, substrate processing, assembly, testing, and environmental screening. Recent production experiences in manufacturing three unique microcircuits containing T r L , C-MOS and ECL devices are discussed. Some problems and possible solutions for hybrid microcircuit reliability F. N. SINNADURAI, K. J. WILSON and D. W. J. B R A C E Microelectronics Journal 11 (1), (1980). Thick film hybrid microcircuits developed for various telecommunications applications and made by various technologies have been examined by overstress testing and by failure analyses, which have yielded a number of promising results and revealed a number of deficiencies. Examples are presented of the poor reliability obtainable not only from low-cost plastic-coated hybrids but also from supposedly superior hermeticallysealed hybrids - degradation occurring because of moisture permeation or desorption, the evolution of deleterious vapours, bond failure due to inter-metallic reactions, and so on. Reliability problems of this sort are often encountered when the technologies or manufacturing standards are not directed towards high reliability applications, despite conformity to accepted quality standards, so demonstrating that approval of manufacturing capability does not necessarily provide realiability assurance. Nevertheless, the reliability prospects for hybrids are quite bright, because problems can and have been overcome - for example, that some circuits even in low-cost encapsulations can approach the highest reliability standards, that the long term stability of thick-film resistors can be better than 1%, that bonding problems can be obviated by using the right interconnection materials and that less hazardous adhesives are avilable for use in hermetic encapsulations. The observations show that while it cannot be claimed that hybrid technologies are inherently reliable, neither does it appear that the opposite is t r u e - potentially high reliabilities beingachievable by exercising intelligent choice.
4.
Discrete Devices
Reversible breakdown voltage collapse in silicon gate-controlled diodes A. RUSU, O. P I E T R A R E A N U and C. BULUCEA Solid-St. Electron. 23, 473 (1980). A reversible breakdown voltage collapse is recorded in the high voltage range of the junction breakdown voltage vs gate voltage characteristic of silicon gate-controlled diodes, which is explained in terms of a spatial switching of the avalanche breakdown within the device structure. The collapse gate voltage is oxide-thickness dependent and is accurately predictable as the avalanche breakdown voltage of the deeply depleted MOS capacitor within the gatecontrolled diode structure. The minimum oxide thickness required for approaching the bulkdetermined breakdown voltage in field-plated planar diodes and transistors is found to range from 0.01 to 5.00 p.m for substrate impurity concentration from 1017to 5 x 101~cm-3, according to a design plot provided in the paper. Development of the thick-film capacitor and its application for hybrid circuit modules K. A B E , A. IKEGAMI, N. SUGISHITA. N. TAGUCHI,
IEEE Trans. Components, Hybrids Mfg Technology. Chmt-2(4), 434
TCR control of NI/Cr resistors R. A. THIEL and E. H. M A U R E R
IEEE Trans. Components, Hybrids Mfg Technology. Chmt-2(4), 467 (December 1979). A detailed study was made to identify the important deposition parameters and to determine how they interact to affect the TCR and sheet resistance of Ni/Cr films deposited on alumina. Wire-fed flash evaporation is described as a simple method fur the control of composition, deposition rate, and thickness. The Ni/Cr ratio, substrate temperature, amount of Au in the Ni/Cr, and the film thickness have major effects on TCR. Deposition rate and pressure and the effect of a Ni barrier layer between the Ni/Cr and the Au conductor were also investigated. The use of a Ni barrier layer degrades the resistor stability somewhat. Approximately 50% Cr is necessary to get the TCR on alumina below about + 100ppmpC, but variation from 50 to 70% has little further effect, while Au in the Ni/Cr depresses the TCR, and can make it negative. Reduced substrate temperature during film condensation and increased film thickness also reduce TCR. Sheet resistance is affected by thickness, Au content and substrate temperature. A particular value for each of the above parameters was found that when used together produced 'zero' TCR and 180f~/square sheet resistance. Influence of the substrate on the electrical properties of thick-film resistors A. CATI'ANEO, L. PIROZZI, B. MORTEN and M. PRUDENZIATI IEEE Trans. Components, Hybrids Mfg Technology. Chmt-3(l), 181 (March 1980). The thick-film technology is widely used for hybrid circuit manufacturing and there is a definite trend to use it in other applications. However, the understanding of the electrical, mechanical, and chemical characteristics of thick-film materials has been approached essentially on an empirical basis, even if many hypotheses have been formulated on their conduction mechanisms. In fact, thick-film resistors present a temperature coefficient of resistance that is negative at lower temperatures and positive at higher temperatures with a TCR vanishing in the range of the room temperature. Recent papers attribute the conduction mechanism to percolation tunneling of electrons through conductive grains embedded in the glassy matrix of the resistor layer. The model assumes that the resistance of the percolation paths dominates the resistance of a network electrically equivalent to the thick-film resistor. With this model a good fitting of the experimental data is obtained. However only data concerning Ru-based resistors screened and fired on 96% alumina substrates were considered. In order to better understand the influence of the ceramic substrates on the electrical and thermal characteristics of thick-film resistors, zirconia, beryllia, and alumina (with different purity) were employed in the present study. The results indicate that the 'substrate effect' plays an important role in ruthenium-based resistors, so that, in order to understand the thick-film conduction mechanisms, it is necessary to take into consideration the 'substrate resistor system' and not to limit the analysis to the film in itself. Of particular interest is the fact that the minimum of the resistance-versus-temperature CUlWevaries for different substrate materials, even if the resistors under test are made with the same resistor series and are characterised by the same sheet resistivity. An equation is proposed that correlates the resistor gauge factor to thermal expansion coefficient of the ceramic substrate. By assuming the validity of a recently proposed model of conduction mechanism in thick film, a new set of equations is proposed that fits the experimental results obtained on resistors screened and fired on substrates of different compositions or with a different content of impurities. 41